The prior art discloses circuits in which the transmission, reception and antenna ports are each connected to an input of a duplexer. In this case, very stringent requirements are placed on the duplexer since it needs to conduct very high transmission powers to the antenna and at the same time needs to conduct signals with a low reception power from the antenna to the receiver port with low losses. An important characteristic for the duplexer circuit is the isolation, which specifies how well a reception port is protected from the high transmission power of a transmission port. Owing to the finite isolation of a real duplexer, a small parasitic component of a transmission signal always passes from the transmission port to the reception port.
U.S. Patent Publication No. 2009/0296790 A1 describes a circuit in which the three ports are interconnected with one another via three 90° hybrids and two ferrite circulators. Such a circuit ensures good isolation. The signal transmitted by the transmission port is first split into two signals which are phase-shifted relative to one another through 90°. These two signals interfere constructively at the antenna port and destructively at the reception port. The circuit described in said document is used in radar technology, in which there are no stringent requirements placed on selection, with the result that this does not play a role here. The selection describes the ratio between the emission power in the passband and the loss outside the passband. Correspondingly, ferrite circulators can be used here which have very poor circulation.
In mobile radio technology, on the other hand, the transmission and reception frequencies are different, with the reception power being very low. In order to be able to process the reception signal, a high degree of selection between the transmission and reception frequencies is critical here.
U.S. Patent Publication No. 2010/0148886 A1 describes a duplexer which has two 90° hybrids. The 90° hybrids are interconnected in such a way that a parasitic signal passes from a transmission port to a reception port on two paths. The signal is phase-shifted through 180° on one of the two signal paths, with the result that signals which pass to the reception port along different paths interfere destructively at the reception port.